Measuring transformer for detecting hydrocarbons in gases

ABSTRACT

A measuring transformer for detecting hydrocarbons in gaseous media with two resistive oxygen sensors whose electrical resistance is essentially independent of temperature with one of the two resistive oxygen sensors being catalytically activated for reducing hydrocarbons.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a measure transformer for detectinghydrocarbons in gases.

DISCUSSION OF THE BACKGROUND

Increasingly strict environmental legislation is forcing automobilemanufacturers to develop and use exhaust emission control systems,usually catalytic converters, with ever better conversion rates in orderto maintain the government-specified maximum values of emitted emissionssuch as nitrogen oxides, carbon monoxide, or unburned hydrocarbons. Atthe same time, it is required that the function of the exhaust emissioncontrol systems be monitored continuously during operation and thatdefective function be indicated, ranging from exceeding the limits as aresult of aging phenomena of the catalyst to a total failure of theλ-probe that controls the combustion stoichiometry. For this so-calledon-board diagnosis (OBD), an exhaust sensor located downstream of theexhaust emission control system is required that monitors the functionof the exhaust emission control system during operation and whose sensorsignal serves as a basis for determining the state of the exhaustemission control system.

With a four-stroke engine operated at λ=1 (1 represents the fuel-airmixture), the emissions are drastically reduced by a three-way catalyst.While it is not difficult to meet the nitrogen oxide and carbon monoxidelimits, theoretically unburned hydrocarbons (HC) pose the greatestproblems. Malfunction of the exhaust emission control system isindicated only when the HC concentration in the exhaust increases.

There are many ways to diagnose an exhaust emission control system.Several patents such as DE 34 13 760, U.S. Pat. No. 5,740,676, U.S. Pat.No. 5,467,594, and DE 42 09 136 for example as well as the literaturereferences [1] and [2] cited as examples propose providing λ-probesupstream and downstream of the catalytic converter. The oxygen storagecapacity and hence indirectly the function of the catalytic convertercan be determined from a comparison of the amplitude fluctuations in theprobe output signals upstream and downstream of the catalytic converter.Such methods are already used in mass production. Another frequentlydiscussed method is diagnosis of the exhaust emission control system bymeans of one or more temperature sensors. In this case, it is thereaction heat resulting from the conversion of the hydrocarbons in theuntreated exhaust that is detected. Examples will be found in [3]-[7] orin DE 42 01 136. Direct determination of the hydrocarbon concentrationin the purified exhaust by means of an HC sensor would be much simplerand more precise than determination of values that depend onlyindirectly on emissions.

Such direct HC sensors can incorporate for example HC measurement bymeans of a surface ionization detector [8], but this method depends to asignificant degree on the gas throughput, the type of hydrocarbons, andthe oxygen content of the exhaust.

Another type of HC sensor is the familiar catalytic sensor (also knownas the pellistor) described here using the example in EP 0 608 122. Forsuch sensors, oxygen is always necessary to burn the hydrocarbons sothat the output signal depends largely upon the oxygen content of theexhaust. In addition, very exact temperature control and measurement arerequired since the electrical resistance of a temperature-dependent partis measured. Therefore, such a sensor principle is unsuited for use inthe exhaust line.

A sensor design that consists of an oxygen generator, oxygen diffusionzone, HC sensor zone, and at least two temperature control zones and istherefore very complex is described in U.S. Pat. No. 5,689,059. Thissensor is suitable for exhaust but requires electrical terminals inconsiderable numbers. In addition, this sensor, which in reality asensor system, requires very complex and costly control and regulatingelectronics so that it cannot be used for the broad mass market.

Hydrocarbon sensors using planar technology are less expensive tomanufacture.

DE 0 046 989 proposes an HC sensor based on tungsten oxide made by theplanar technique which can be used only at room temperature.

Pt-MOSiC sensors based on silicon carbide are proposed in [9] for use inmotor vehicles. However, the operating mechanism is not easy tounderstand and the signals are dependent not only on the hydrocarbon butalso on oxygen and temperature. Manufacture of planar structures on SiCis also costly and therefore cannot be used for the motor vehicle massmarket.

Widely used, inexpensive sensors are made on a ceramic substrate fromSnO₂. Examples include EP 0 444 753 or EP 0 603 945. In this sensorprinciple, the electrical sensor resistance changes with the HCconcentration in the gas. Sensors of this type are used in large numbersas sensitive elements in gas warning systems and their functionalmechanism is widely known. An attempt to use such sensors in anautomobile is described in [10] and [11]. Unfortunately, these sensorslose their gas-sensitive properties at temperatures above severalhundred degrees Celsius and change their resistance only with the oxygenpartial pressure of the gas. The long-term stability of these sensors isnot guaranteed either.

Resistive sensors based on metal oxides which have been proposed morefrequently as an oxygen-detecting element but not as an HC sensor arelikewise manufactured using planar technology and are suitable for usein exhaust. In the resistive principle, the electrical resistance of thesensitive material is used as a measured value. For example, DE 37 23051 proposes doped titanates, zirconates, or stanates as resistiveoxygen-sensitive materials which are applied according to DE 37 23 052using thick film technology to a ceramic substrate. DE 42 02 146, DE 4244 723, and DE 43 25 183 propose compositiones based on cupratesmanufactured using thick film technology as oxygen-sensitive materials.DE 44 18 054 mentions lanthanum ferrites doped with alkaline earths forthe same purpose. Such multiple metal oxides which are usually presentin the perovskite structure have the advantage of increased chemicalstability and greater long-term stability over the sensors made ofsimple metal oxides, TiO₂ [10] for example, that have been in use for along time. All of these oxygen sensors however have a typicaltemperature curve of the electrical resistance according to anexponential function typical for semiconducting metal oxides, in otherwords the sensor output signal depends not only on the oxygen partialpressure of the exhaust but on the sensor temperature as well. Hence,for such oxygen exhaust sensors, an exact temperature measurement islinked to a costly electronic regulation or a reference that is notexposed to the exhaust and is kept at a constant temperature must beintegrated in the substrate which is also expensive and leads toproblems with long-term stability.

The series connection of two resistive oxygen sensors which in additionto their oxygen dependence have a temperature dependence with differenttemperature coefficients of the specific electrical resistance, isproposed in DE 38 33 295. In addition, a compensating resistance mustalso be included. However, in this method, despite the high costs, atemperature independence of the sensor resistance can be achieved onlyin a very narrowly delimited oxygen partial pressure range.

A resistive oxygen sensor that is temperature-independent only at acertain oxygen partial pressure is described in a portion of EP 0 062994. In DE 19 744 316, an oxygen sensor is proposed made of a materialin which the oxygen partial pressure range of temperature independencecan be varied by deliberate variation (doping) of the layer material.

Typical HC sensors manufactured using planar technology arecharacterized by the following typical arrangement. On the underside ofan electrically insulating substrate a heater and/or a temperaturemeasuring device in the form of a resistance thermometer is mounted.Then, on the substrate surface, an electrode structure is provided thatmeets the special requirements and a functional layer is mounted on topof this structure.

In EP 0 426 989 and in [12], the electrode structure has a so-calledinterdigital capacitor arrangement (IDC). Zeolites are proposed as thefunctional layer. Dependent on the temperature, the complex electricalresistance of the functional layer varies very selectively in this casewith the hydrocarbon concentration in a gas. In [13] Ga₂O₃ is proposedas the material for a resistive planar HC sensor. However, a significantdependence of the sensor output signal on temperature is reported hereas well. This sensor reacts to oxygen [12] especially above 900° C.

In another method for producing planar sensors, a ZrO₂ layer is appliedto the substrate and two different electrodes with different electricalpotentials are added. The differential voltage between the two differentelectrodes is then the measurement signal. This method is described ingreat detail in [14]. A variation is described in U.S. Pat. No.5,352,353and DE 41 02 741 and DE 41 09 516. It is readily apparent that theoutput signal from such sensors depends to a large extent upontemperature and naturally on oxygen partial pressure of the exhaust aswell. Additional cross sensitivities to hydrogen for example, are alsopresent.

In DE 42 28 052, a sensor is described that consists of a combination oftwo individual resistive oxygen sensors, with one of the two sensorsbeing provided with a catalytically active coating. As a result, theincompleteness of the combustion in the engine can be determined fromthe differential signal of the two sensor elements. Such a sensor hasthe disadvantage that it is not temperature-compensated, in other wordsas in all resistive oxygen sensors, the output signal is primarilytemperature-dependent and depends only secondarily on gas concentration.

To remedy this situation, DE 195 31 202 proposes a bridge arrangementconsisting of at least four individual sensor elements of which twoare—and two are p-conducting, arranged so that one branch of the bridgeis activated catalytically and that an—and a p-conducting sensor arearranged in each branch of the bridge circuit. In addition to theconsiderable cost and technical problems involved, in applying at leastfour different materials in addition to the electrical terminals inlayers on a substrate compatibly with one another, this system does notproduce a sensor that is temperature-independent for all oxygen partialpressure ranges.

The disadvantage of all planar HC sensors is the considerabletemperature dependence of the sensor output signal which requires eitherexact temperature regulation or exact temperature measurement followedby electronic compensation of the signal or the materials that differonly by a cumbersome arrangement of different materials on a support canbe partially compensated.

SUMMARY OF THE INVENTION

Hence, the goal of the present invention is to provide a measuringtransformer for HC detection which eliminates the stated disadvantagesof the prior art, especially temperature dependence.

This goal is achieved by the measuring transformer according to claim 1.Advantageous embodiments of the invention as well as specialapplications of the measuring transformer according to the invention arethe subject of additional claims.

According to the invention, two resistive oxygen sensors are provided,one of them provided with a catalytically active layer for reducinghydrocarbons. Both oxygen sensors are characterized by an essentiallytemperature-independent characteristic. The electrical resistances ofthe two sensors are measured and both the hydrocarbon concentration andthe oxygen partial pressure of the gas to be analyzed can be determinedfrom the two measured values. Since the oxygen partial pressure ismeasured as well, the available oxygen dependence of the catalyticallynon-activated oxygen sensor can be calculated.

Both oxygen sensors are held at the same working temperature. As aresult of the temperature independence of the sensor output signal, onlylimited requirements need to be imposed on temperature measurement andregulation. Depending on the desired accuracy, they can even beeliminated. A resistance heater can be provided as a heater for example.It is also possible to provide heating by hot gases which are availablein the vicinity of the hydrocarbon detection, for example the exhaustgases from a furnace or an internal combustion engine.

The measurement transformer according to the invention is preferablymade using thick film or thin film technology with the two oxygensensors being mounted on an electrically insulating substrate.

Preferred areas of application for the measurement transformer accordingto the invention are:

Measurement of the hydrocarbon concentration in the exhaust from afurnace or heating system. In addition, the output signal from thenon-activated resistive oxygen sensor can be used to detect the oxygencontent in the exhaust from the furnace or heating system.

Measurement of the concentration of flammable gases, especiallyhydrocarbons, in the ambient air.

Measurement of the hydrocarbon concentration in the exhaust from aninternal combustion engine. The output signal of the activated resistiveoxygen sensor can be used in addition for detecting the fuel-air mixturein the exhaust from the engine.

Diagnosis of an exhaust emission purification system in the exhaust froman internal combustion engine. In addition, the output signal from theactivated resistive oxygen sensor can be used to detect the oxygen-airmixture of the internal combustion engine.

Performing mixture control in an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described below using a few preferredembodiments as examples, with reference to drawings. The embodimentsdescribed serve only to explain the invention and are not intended toimply that they represent a limitation of the invention to these specialembodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the typical curve of the electrical resistance of atemperature-independent resistive oxygen sensor using thick filmtechnology which can be used in the measuring transformer according tothe invention;

FIG. 2 shows a sample embodiment of the measuring transformer accordingto the invention in a schematic view;

FIG. 3 shows the sensitivity of a measuring transformer according to theinvention and its cross sensitivity to hydrogen, carbon monoxide, andnitrogen monoxide;

FIG. 4 shows the technical implementation of a measuring transformeraccording to the invention as in FIG. 2.

In EP 0 062 994, a resistive oxygen sensor that is temperatureindependent at a certain oxygen partial pressure is shown. In DE 197 44316, an oxygen sensor is proposed in which the oxygen partial pressurerange during temperature independence can be varied by changing thelayer material. A typical curve from DE 197 44 316 is shown in FIG. 1.The sensor resistance varies almost independently of temperature withoxygen partial pressure (pO₂). The dependence is approximately R˜pO₂^(−a), with a≈⅕. For the function of the electrical resistance over theoxygen partial pressure, Equation 1 applies approximately:

R_(x)≈R_(x,0)×pO₂ ^(−⅕)  (1)

Here, the subscript X assumes the values 1 and 2. Its significance isexplained below. The value of the prefactor R_(x,0) depends on thegeometry of the sensor.

According to the invention, two resistances, both of which can bedescribed by Equation 1, are combined with one another as shown in FIG.2 for example. In the embodiment shown there, the measuring transformeraccording to the invention has four terminals for measuring the tworesistances R1, R2. However, three terminals will suffice if the tworesistances R1 and R2 have a common terminal. The second resistance(subscript X=2) is activated catalytically according to the invention sothat hydrocarbons arriving at it react immediately. The oxygen partialpressure at its surface is then reduced. The heat released during thereaction does not affect the sensor resistance, however, since theresistance does not depend upon the temperature. The non-activatedresistance (subscript X=1) accordingly measures the free oxygen whilethe activated resistance measures the hydrocarbon content presentfollowing the reaction of the hydrocarbons with oxygen. The combustionC_(m)H_(n) can serve as an example: The reaction of the hydrocarbonsrequires oxygen according to Equation 2:${{C_{m}H_{n}} + {\left( {m + \frac{n}{4}} \right)O_{2}}}->{{mCO}_{2} + {\frac{n}{2}H_{2}O}}$

For propane (m=3, n=8), eight oxygen molecules would be consumed perC₃H₈ molecule if complete reaction can take place on the surface of thesensor. The oxygen partial pressure is defined over part z of the oxygenmolecules on the total number of molecules according to Equation 3:

pO₂=z×p_(total)  (3)

In determining the hydrocarbon concentration, the basis used is that theoxygen partial pressure on catalytically active resistance 2 ispO_(2.2),$\left\lbrack {z - {\left( {m + \frac{n}{4}} \right) \times c_{HC}}} \right\rbrack \times p$

total and is therefore lower than the oxygen partial pressure on thenon-activated resistance 1, pO_(2.1). For the relationship between theoxygen partial pressures and the concentration of the hydrocarbonsC_(HC), Equation 4 therefore applies:${pO}_{2.2} = {\left( {1 - \frac{\left( {m + \frac{n}{4}} \right) \times c_{HC}}{z}} \right) \times {pO}_{2.1}}$

With m and n known, from knowledge of the two oxygen partial pressures,the hydrocarbon concentration in the gas can be determined. Thevariables m and n and comparable values that describe the oxygenrequirement for combustion can be determined from experiments orcalculated with a known exhaust composition.

A simple evaluation possibility is offered by the formation of the twosensor resistances R1 and R2, with $V = \frac{R_{1}}{R_{2}}$

for the relationship$V = {\frac{R_{1.0}}{R_{2.0}} \times \left( {1 - \frac{\left( {m + \frac{n}{4}} \right) \times c_{HC}}{z}} \right)^{\frac{1}{5}}}$

A curve of this kind is plotted in FIG. 3. It is clear how thesensitivity to hydrocarbons (curve 1, solid) stands out from the crosssensitivities to hydrogen and carbon monoxide (curve 2, dashed) ornitrogen monoxide (curve 3, dotted).

However, it is not necessary to plot the ratio; instead, thedifferential signal between the two resistances or another possiblemethod for evaluating the different resistance curves may be selected.

In the above explanation of the measurement principle, it was assumedfor the sake of simplification that only one specific hydrocarbon ispresent in the gas to be investigated. The invention is not limited tothis. Even with several different hydrocarbons, it offers usable resultsabout the presence of hydrocarbons. One application for this is the useof the measuring transformer according to the invention in order todiagnose the function of an exhaust emission control system connectedupstream of an internal combustion engine.

As already mentioned, individual signals from the two oxygen sensors canalso be used for separate evaluation in order to obtain additionalinformation through the oxygen content for example.

A material suitable for both oxygen sensors is metal oxides, especiallythose that have a perovskite (ABO₃) structure; both the A-locations andthe B-locations can be occupied by more than one type of ion. A typicalrepresentative is multiply doped Sr(TiFe)O₃.

It should also be pointed out that the two oxygen sensors do not have tobe made of the same material.

The sketch of a technical implementation of a measuring transformeraccording to the invention using thick film technology is shown in FIG.4. A heating and measurement resistance structure made of platinum forexample is mounted on underside 2 of a substrate 10 which consists inthis simplified sketch of leads and contact surfaces 22 and 24 with animpedance as low as possible as well as a zone with a high resistance30. In this range, the sensor is heated. On the top 4 of substrate 10there are three leads 12, 14, and 16 which likewise have a contact areaapplied to them. They consist of an electrically conducting materialwhich is ideally catalytically inactive. According to the invention, tworesistive temperature-independent oxygen sensors 18 and 20 are mountedon the leads, with one of the two oxygen sensors being additionallyprovided with a catalytically active and electrically nonconductinglayer.

To produce a measuring transformer according to the invention on thebasis of complex metal oxides, the oxides, carbonates, and/oroxycarbonates of the metals that occur in the sensitive material areadded in stoichiometric ratio, mixed intimately, ground up in an organicsolvent, dried, and burned and the metal oxide powder thus obtained isprocessed to form a paste. This paste is applied to a preferablyelectrically insulating substrate and burned. The electrodes required tomeasure the electrical resistance are attached either before or afterthe metal oxide paste is burned.

In order to make a measuring transformer according to FIG. 4, one canproceed in detail as follows. First of all, a ceramic powder that hasthe desired sensitive properties is produced. For this purpose, thestarting materials (oxides, carbonates, nitrates, or oxycarbonates forexample) are measured to obtain the desired stoichiometric ratio. Atypical batch of approximately 50 g raw powder is then processed in agrinder together with a grinding medium which can be a solvent such ascyclohexane or isopropanol for example, with grinding balls (forexample, made of agate, 10 mm in diameter, 50 in number or a smallerball diameter with a correspondingly larger number of balls) and mixedfor 1-4 hours in a planetary ball mill. The ground material thus mixedis dried, separated from the balls, loaded into a crucible, and burnedin a furnace in an atmosphere of air at 1200° C. for 15 hours. Thecooled powder then has the desired material composition as can be provenfor example by X-ray diffractometry. Powder burned in this way must becomminuted further by means of another grinding step in order to havethe powder grain size distribution suitable for the remainder of themanufacturing process. A typical grinding process as described above canbe carried out but 7-10 balls each having a diameter of 20 mm should bechosen. The powder can be comminuted in an attrition mill or in anannular ball mill. The powder, dried and separated from the balls, canthen be processed further to form a paste that can be used for screenprinting. Then, suitable connectors (made of gold or platinum forexample, are printed on a substrate (for example, made of Al₂O₃ or ZrO₂)by screen printing in order to be able to measure the sensor resistance.These connectors are typically stoved in an atmosphere of air. Then thesensor layers are printed and likewise stoved. A heating layer can alsobe added to the back of the sensor. One of the two oxygen sensors thusproduced is given a gas permeable, porous, catalytically active, butelectrically nonconducting coating. Leads are attached to the electrodesof the measuring connectors and the heating connectors. The design canbe as shown in FIG. 4. A suitable housing provided with electrical leadsensures mechanical stability and protects the sensor.

References

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What is claimed is:
 1. A measuring transformer for detectinghydrocarbons in gaseous media, said transformer comprising: tworesistive oxygen sensors whose electrical resistance is essentiallytemperature-independent, wherein one of said two resistive oxygensensors is catalytically activated to reduce hydrocarbons; and a devicefor measuring said electrical resistance of each of said two oxygensensors; said oxygen sensors each comprise one of metal oxides presentin perovskite structure or a structure that resembles perovskite,wherein the metal oxide consists of a compound with one of the followingcompositions: (Sr_(1−n)N_(n))_(1−a)M_(a)Ti_(1−z)Fe_(z)O_(3−d) or(Sr_(1−n)N_(n))(Ti_(1−z)Fe_(z))_(1−b)M′_(b)O_(3−d) or(Sr_(1−n)N_(n))(Ti_(1−z)Fe_(z))_(1−c)M″_(c)O_(3−d) or(Sr_(1−n)N_(n))_(1−e)M′″_(e)Ti_(1−z)Fe_(z)O_(3−d)  where, N is strontium(Sr), barium (Ba), calcium (Ca), magnesium (Mg), zinc (Zn), cadmium(Cd), mercury (Hg), lead (Pb), or radium (Ra) or a mixture of two ormore thereof; Sr is strontium; M stands for an element of thelanthanides (numbers 57 to 71 in the periodic system of the elements) orfor yttrium (Y), indium (In), or thallium (Tl) or for a mixture of twoor more thereof; Ti is titanium; Fe is iron; O is oxygen; n is a numberbetween zero and one; a, b, c or e are numbers larger than or equal tozero and smaller than or equal to one-half; z is a number larger than orequal to one-tenth and smaller than or equal to six-tenths; d is theoxygen deficit that results depending on the composition from theelectroneutrality condition; M′ is phosphorus (P), vanadium (V),chromium (Cr), manganese (Mn),arsenic (As), niobium (Nb), antimony (Sb),tantalum (Ta), molybdenum (Mo), tellurium (Te), or tungsten (W) or amixture thereof; M″ ist aluminum (Al), scandium (Sc), gallium (Ga),cobalt (Co), nickel (Ni) or a mixture of two or more of these elements;M′″ is lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium(Cs), copper (Cu), or silver (Ag) or a mixture thereof.
 2. The measuringtransformer according to claim 1, further including a controlled devicefor providing and maintain a specific working temperature for said twooxygen sensors.
 3. The measuring transformer according to claim 1,wherein said the two oxygen sensors are positioned on an electricallyinsulating substrate by the use of at least one of a thick film and athin film technology.
 4. The measuring transformer according to claim 1,wherein a portion of the titanium is replaced by another tetravalentelement, especially silicon (Si), germanium (Ge), zirconium (Zr), tin(Sn), cerium (Ce), or hafnium (Hf).
 5. The measuring transformeraccording to claim 1, wherein the perovskite structure is of the formABO₃ and wherein the stoichiometry of the perovskite structure metaloxides is adjusted so that the atomic ratio of ions at A-locations toions at B-locations is between 0.7 and 1.3.
 6. The measuring transformeraccording to claim 1, wherein the two resistive oxygen sensors consistof different materials.